Dental implants are used to support one or more artificial teeth. Implants can help support, stabilize and retain full dentures. Suction aids retention of upper full dentures, but suction is generally inadequate for retention of lower full dentures.
At present there are two kinds of dentures, generally termed “hard dentures” and “soft dentures”. Hard dentures are made out of any type of plastic and are unlined. Soft dentures have a rigid structure with a liner made of a soft, cushioning material, such as silicone, in the inner channel over the patient's gingival tissue. Some prior known implants systems for dentures use a ball shaped abutment that projects from each implant. Hard dentures are retained by screws that extend through the denture and into the implants. An alternative system for retaining hard dentures has a cap or holder imbedded in the channel of the denture. The cap has a rubber or elastomeric “O” ring that snaps over a ball shaped abutment on the implant. The implants support the hard dentures with these retention systems and the forces of chewing are on the implants.
The liner in a soft denture is intended to cushion and support the denture on the gingival tissues. One known system for retaining a soft denture has cavities opening into the inner channel. The liner extends into and lines the cavities, and shaped abutments on the implants protrude into the cavities to retain the denture. These systems require complex cavity and abutment shapes in order to provide sufficient retention.
There are two main types of implant systems currently in use. The first type uses what is generally known as a large diameter implant and is greater than 3 mm in diameter. This system is generally a two piece system that includes an implant with an internally threaded cavity and an abutment that threads into the cavity in the implant. The large diameter implant system is very stable after it fuses to the bone, allows precise selection of the abutment height after the implant is installed, and after years of use the abutment neck height can be changed to accommodate receding tissues. However, the large diameter implant is more difficult to place by the General Dentist and is most often placed by a specialist, is more expensive to make and install, has a higher risk of damage to the nerves in the bone, and there is usually a 2 to 3 month wait for the bone to fuse to the implant.
The second type of implant system uses what is generally known as a mini or small diameter implant with the implant diameter being less than about 3 mm. Since the diameter of the implant is substantially too small for a cavity with internal threads, in this system the implant and abutment are generally one piece. The mini implant is easy for the General Dentist to install, is less expensive to make and install, is usually easier to place, has a lower risk of damage to the nerves in the bone, is screwed tightly into the bone without invasive surgery and can be loaded immediately. However, the mini implant is smaller and thus less stable under heavy loads, and can be placed by less trained dentists, resulting in the risk of abuse and misplacement. The one piece mini implant is harder to get the correct length, and can protrude too far into the denture base, such that a soft denture is supported by the implant instead of the gingival tissue.
U.S. Pat. No. 4,787,851 to Kusano, et al. discloses a denture having an inner channel with an elastomeric liner and ball shaped abutments that project into the liner. U.S. Pat. No. 2,112,007 to Adams discloses a denture having an inner channel with snap ring cup sockets and ball shaped abutments that project into the sockets. U.S. Pat. No. 5,049,072 to Lueschen discloses a denture having an inner channel with sockets, o-rings and ball shaped abutments that project into the sockets.
There is a need for a denture retention system that sufficiently retains the denture while allowing the gingival tissue to support the denture. In such a system there is the further need of being able to precisely select the abutment height and to adjust the abutment height to accommodate receding tissues, with both large diameter and mini implants.